In order to satisfy statutory standards, the optical assembly in such headlamps must be pivotable relative to the glass to allow the orientation of the optical axis to be adjusted relative to the plane of the ground on which the vehicle stands, i.e. a plane defined by contact between the wheels of the vehicle and the ground, and/or relative to a plane perpendicular to said plane and defining a longitudinal mid plane of the vehicle.
To this end, headlamps of this type are generally made by providing means which connect the optical assembly to the glass by defining two pivot axes, one of which is approximately perpendicular to the longitudinal mid plane of the vehicle, i.e. approximately parallel to the plane of the ground on which the vehicle stands, and the other of which is approximately perpendicular to the plane of the ground on which the vehicle stands, i.e. is approximately parallel to the longitudinal mid plane of the vehicle. Means are also provided for adjusting the position of the optical assembly relative to the glass by pivoting about each of said pivot axes.
Adjusting the orientation of the optical axis relative to the plane of the ground on which the vehicle stands and relative to the longitudinal mid plane of the vehicle may be performed without difficulty after the headlamp has been mounted on the vehicle, i.e. after the glass has been integrated into the vehicle bodywork. However, to do this it is necessary to use an optical method of adjustment consisting in emitting a beam from said headlamp and in projecting said beam in the form of a light spot on a screen which occupies a predetermined position relative to the vehicle. The adjustment of the headlamp orientation can be seen on the screen relative to both of said planes. If the adjustment appears to be wrong, the optical assembly is made to pivot relative to the glass by acting on said adjustment means so that it rotates about one or both of the pivot axes, and this is continued until the light spot formed on the screen by the beam occupies the correct position, indicating that the optical axis of the headlamp is oriented in accordance with the regulations laid down. Throughout the adjustment process, the relative positions of the light spot on the screen and a mark displaying the correct position for the light spot on the screen enable the operator to observe how the adjustment process is proceeding and also indicate which of the adjustment means should next be used to improve the adjustment.
Unfortunately, this type of headlamp is incompatible with "mechanical" adjustment means which detect wrong adjustment of the orientation of a headlamp's optical axis by detecting a possible difference between the orientation of a plane which is fixed relative to the glass and a predetermined orientation, as is applicable to the system described by U.S. standard SAE J602 OCT80 and as described in U.S Pat. No. 2,997,914, for example. These systems seek to adjust headlamps whose glass and optical assembly are directly fixed together such that orientation of a plane fixed to the glass is representative of the orientation of the optical axis. In other words, the orientation of such a plane which is fixed relative to the glass is not necessarily representative of the orientation of the optical axis associated therewith in headlamps of the type to which the present invention is applicable.
Preferred embodiments of the present invention enable mechanical means to be used for adjusting the orientation of the optical axis of a headlamp optical assembly when the headlamp glass is fixed to the vehicle bodywork, e.g. as required by the above-mentioned U.S standard and U.S patent.